The present invention relates generally to a video output circuit of a wide bandwidth designed for a CRT (Cathode Ray Tube) display device or the like.
For the video output circuit developed in these years for practical applications to CRT displays of super high resolution, there is required as high an output power as 100 V.sub.pp with a frequency on the order of 100 MHz.
In such video output circuit, a video output terminal thereof is inevitably accompanied with a stray capacitor of a finite value. As a consequence, power consumption of the video output circuit increases in proportion to a frequency bandwidth and a voltage amplitude as required. As an attempt for suppressing the power consumption from increasing, there have heretofore been proposed a variety of high-frequency peaking circuits, typical ones of which are shown in FIGS. 2A, 2B and 2C of the accompanying drawings, respectively. More specifically, FIG. 2A shows a shunt circuit of a capacitor (C) and a resistor (R) including no peaking circuit. In this figure, reference numeral 1 denotes a current source for supplying a current of magnitude I, numeral 2 denotes a resistor having a resistance value R, and numeral 3 denotes a capacitor having a value of C. The output voltage of this shunt circuit is indicated by a symbol E. Representing a 3dB bandwidth by f.sub.c, there exists among the parameters C, R and f.sub.c a relation given by an expression inserted in FIG. 2A.
FIG. 2B shows a peaking circuit whose order is equal to 3 (a peaking circuit of second order, to say in another way), in which reference numerals 4 and 5 denotes capacitors having values C.sub.1 and C.sub.2, respectively, and 6 denotes an inductor having a value L. In order to realize a so-called Butterworth maximally-flat frequency characteristic in this circuit, the parameters C.sub.1, L, C.sub.2, R and f.sub.c are so selected as to satisfy the condition given by the expression inserted in FIG. 2B.
Finally, FIG. 2C shows a bridged-T-type peaking circuit, details of which is disclosed in U.S. Pat. No. 4,528,520 entitled "WIDE BAND HIGH OUTPUT AMPLIFIER USING A POWER FIELD EFFECT TRANSISTOR AS AN OUTPUT STAGE". In the figure, reference numeral 7 denotes an output capacitor of a value C.sub.0, and numerals 9 and 10 denote stray capacitors of values C.sub.s1 and C.sub.s2, respectively.
When a transfer impedance of each of the circuit shown in FIGS. 2A and 2B is represented by T(.ident.E/I), frequency characteristics of these circuits can be determined in accordance with the expressions (1), (3) and (5) mentioned below. In this conjunction, it is considered to be convenient and reasonable to define a merit index (M) of a peaking circuit in terms of a product of an overall capacitor value, a load resistor value, the 3 dB bandwidth and 2.pi..
In accordance with the above definition, the merit index M of the circuit shown in FIG. 2A is "1" (one) in accordance with the expression (2) mentioned below.
On the other hand, the merit index M of the peaking circuit shown in FIG. 2B is equal to "2", as determined in accordance with the expression (4) also mentioned below.
In the case of the peaking circuit shown in FIG. 2C, the merit index M is equal to 2.sqroot.2 on the assumption that the stray capacitors C.sub.s1 and C.sub.s2 are absent, as can be seen from the undermentioned expression (6). EQU .vertline.T/R.vertline.={1+(f/f.sub.c).sup.2 }.sup.-0.5 ( 1) EQU M=2.pi.f.sub.c CR=1 (2) EQU .vertline.T/R.vertline.={1+(f/f.sub.c).sup.6 }.sup.-0.5 ( 3) EQU M=2.pi.f.sub.c (C.sub.1 +C.sub.2)R=2 (4) EQU .vertline.T/R.vertline.={1+(f/f.sub.c).sup.4 }.sup.-0.5 ( 5) EQU M.ident.2.pi.f.sub.c C.sub.0 R=2.sqroot.2 (6)
As is apparent from the above, the prior art circuits shown in FIGS. 2A to 2C suffer limitation that the merit index M can not exceed three.